1. Field of the Invention
The present invention relates in general to the field of hologram production and, more particularly, to hologram production where a recording laser beam polarization is adjusted.
2. Description of the Related Art
One-step hologram (including holographic stereogram) production technology has been used to satisfactorily record holograms in holographic recording materials without the traditional step of creating preliminary holograms. Both computer image holograms and non-computer image holograms may be produced by such one-step technology. In some one-step systems, computer processed images of objects or computer models of objects allow the respective system to build a hologram from a number of contiguous, small, elemental pieces known as elemental holograms or hogels. To record each hogel on holographic recording material, an object beam is conditioned through the rendered image and interfered with by a reference beam. Examples of techniques for one-step hologram production can be found in the U.S. Patent Application entitled xe2x80x9cMethod and Apparatus for Recording One-Step, Full-Color, Full-Parallax, Holographic Stereograms,xe2x80x9d Ser. No. 09/098,581, naming Michael A. Klug, Mark E. Holzbach, and Alejandro J. Ferdman as inventors, and filed on Jun. 17, 1998, which is hereby incorporated by reference herein in its entirety.
In general, the holographic recording materials used in the fabrication of holograms include photopolymerizable compositions, dichromated gelatin, and silver halide emulsions. These holographic recording materials are typically placed on a glass or plastic substrate before being used in hologram production equipment (e.g., a xe2x80x9cholographic printerxe2x80x9d). Glass is a particularly useful substrate because of its good optical properties (e.g., high transmission, low distortion, low birefringence) and because of other beneficial mechanical properties including flatness, dimensional stability, scratch resistance, and chemical inertness.
FIG. 1A illustrates a typical holographic film 100 based on a holographic recording material layer 120 such as a photopolymerizable composition. Although a variety of different types of holographic recording material can be used with the techniques discussed herein, including photopolymerizable compositions, dichromated gelatin, and silver halide emulsions, holographic recording material layer 120 is preferably formed from a photopolymer. Photopolymers include a wide range of materials that undergo physical, chemical, or optical changes through selective polymerization when exposed to light. Changes in the photopolymer""s refractive index, transparency, adhesion, and/or solubility differentiate light and dark regions when these materials are exposed to an activating light source. Photopolymers capable of recording volume phase holograms include those developed by Canon Incorporated (based on polyvinyl carbazole), Polaroid Corporation (based on polyethylene amine/acrylate), and E. I. du Pont de Nemours and Company (based on polyvinyl acetate and polymethyl methacrylate). Those having ordinary skill in the art will readily recognize that a variety of different photopolymer compositions can be used in the practice of the inventions described herein. Nevertheless, preferred photopolymer films are provided by E. I. du Pont de Nemours and Company under the trade designations, for example, OmniDex(trademark) 706, OmniDex(trademark) 801, HRF-800X001-15, HRF-750X, HRF-700X, HRF-600X, and the like.
FIG. 1A illustrates a typical photopolymer holographic film 100 as it is delivered from the film""s manufacturer. Holographic film 100 includes a holographic recording material layer 120, a base sheet 110, and a cover sheet 130. Base sheet 110 and cover sheet 130 provide protection to holographic recording material layer 120, as well as dimensional stability to assist in the handling of the holographic film. Because of its protective and/or dimensional stability functions, base sheet 110 (and/or cover sheet 130) can be referred to as a xe2x80x9cfilm substrate.xe2x80x9d As will be seen below, this film substrate is distinct from substrate 140 as shown in FIG. 1C. Base sheet 110 and cover sheet 130 are typically formed from polymer films, such as polyethylene, polypropylene, cellulose, polyvinyl chloride (PVC), and polyethylene terephthalate (PET). Although not shown, holographic film 100 can include additional layers, such as a barrier layer used, for example, to prevent interlayer diffusion of sensitizing dyes, and to provide protection from oxygen during exposure.
In preparation for placement of the holographic recording material layer 120 on a substrate, cover sheet 130 is removed from holographic film 100 as shown in FIG. 1B. The remaining portions of holographic film 100 (i.e., a holographic recording material layer 120, and a base sheet 110) are then placed on glass or plastic substrate 140, as illustrated in FIG. 1C. The natural tackiness of recording material layer 120 usually is sufficient to bind recording material layer 120 to substrate 140. Because at least some of the light used to record a hologram in holographic recording material layer 120 typically passes through base sheet 110, base sheet 110 preferably has good optical and material qualities including, for example, low scatter, flatness, low or no birefringence, mar-resistance, strength, and suitable thickness.
However, typical steps in the manufacturing process (and variations in the manufacturing process generally) for materials used for base sheet 110 can lead to at least one undesirable optical property, changes in birefringence from one portion of the material to another. For example, in the manufacturing of PET (e.g. Mylar(copyright) from E. I. du Pont de Nemours and Company) a common material used for base sheet 110, molten PET is extruded onto a chill roll drum to form the initial film. The film is first stretched in the direction of the extruded film path (i.e., the xe2x80x9cmachine directionxe2x80x9d or the xe2x80x9cdown-web directionxe2x80x9d) using a series of rollers running at increasingly faster speeds. The film is then stretched in a transverse direction using, for example, a tenter, that pulls the film at right angles to the machine direction. Stretching rearranges the PET molecules into an orderly structure in order to improve the film""s mechanical properties. Nevertheless, minor variations in this process, or the operation of the equipment used in this process, can lead to variation of the orientation of the film""s molecules, which in turn can cause changes in birefringence from one portion of the film to another.
The birefringence of base sheet 110 affects the quality of polarized light (e.g., the polarized laser light from a reference or object beam) used in holographic recording. In general, birefringent materials have different indices of refraction for different directions of light transmitted therethrough. Materials typically used for base sheet 110, such as PET) can be classified as uniaxial or biaxial materials. Uniaxial films usually have two indices of refraction, one in the direction of stretch and the other which is generally perpendicular to the stretched direction. Biaxial materials typically have three indices of refraction: one in the direction of stretch or linear extent of the film material and generally in the plane of the material; a second perpendicular to the first and also in the generally in the plane of the material; and a third index of refraction looking through the material at an edge view of it. In these materials there are one or more axes along which there is no change in the index of refraction exhibited by the material. Those axes typically are referred to as the optical axes or optic axes, and generally define at least one dominant polarization direction.
If the polarization of the laser light transmitted through the film substrate is aligned with the dominant polarization direction of the film substrate, the modulation depth of the recorded hologram is maximized. Correct alignment between the recording beams and the film substrate allows for efficient holographic recording; a minimum amount of light is absorbed or reflected by the film substrate, allowing maximum coherent light exposure for the holographic recording.
FIG. 2 illustrates the process of recording interference patterns in the holographic recording material layer 120. To accomplish this task, any number of different recording apparatus and techniques can be used, such as the apparatus and techniques for one-step hologram production found in the aforementioned U.S. patent application Ser. No. 09/098,581. The disclosure of U.S. patent application Ser. No. 09/098,581 is merely illustrative, and those having ordinary skill in the art will readily recognize that a variety of different schemes can be used to produce holograms. Reference beam 200 and object beam 210 are coherent light beams typically formed from the same original coherent light source (i.e., a laser whose output beam is split into two separate beams), and typically having the same polarization. The interference pattern created by the interference of reference beam 200 and object bean 210 is recorded in holographic recording material layer 120. Previously recorded holographic elements (hogels) 220 demonstrate that holographic elements are recorded in discrete locations within holographic recording material layer 120, with the substrate 140 (or beams 200 and 210) being repositioned after each recording step so that multiple holographic elements are recorded throughout holographic recording material layer 120. In the example shown, glass or plastic substrate 140 is adjusted in the direction of indexing direction 230 in order to record respective holographic elements.
If the dominant polarization direction of base sheet 110 is relatively constant, the recording laser beam polarizations can be aligned in a specific direction allowing for efficient holographic recording. However and as noted above, changes in the dominant polarization direction of the base sheet do occur, and those changes can be substantially systematic and predictable, or relatively random and unpredictable. For example, the dominant polarization direction across the transverse direction base sheet 110 can vary by 20xc2x0 or more. The changes in the dominant polarization direction of the base sheet lead to unevenness in the holographic recording (leading to an image defect) that becomes particularly noticeable with large-scale and full-color holographic images.
Accordingly, it is desirable to overcome the adverse effects caused by changes in the dominant polarization direction within the film substrates used for recording holograms.
It has been discovered that systems and methods for adjusting the polarization of one or more recording laser beams can correct (in whole or in part) for the changes in the polarization of the recording laser beam(s) caused by changes in the dominant polarization direction within the film substrates used for recording holograms. Using information about the dominant polarization direction of portions of holographic film substrate, a polarization adjusting device can be used to adjust the polarization of the recording laser beam(s) to compensate for the effects that changes in the dominant polarization direction of the holographic film substrate have on the recording laser beam(s) and thus the recorded hologram.
Accordingly, one aspect of the present invention provides a system for adjusting the polarization of a recording laser beam. The system includes a polarization adjusting device and a controller coupled to the polarization adjusting device. The controller is operable to control the polarization adjusting device according to information about a dominant polarization direction of a first portion of a holographic film substrate.
Another aspect of the invention provides a method. A first portion of a holographic film substrate is measured to determine information about a dominant polarization direction of the first portion of the holographic film substrate. The polarization of a recording laser beam is adjusted based on the information about a dominant polarization direction of the first portion of the holographic film substrate.
The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. As will also be apparent to one of skill in the art, the operations disclosed herein may be implemented in a number of ways, and such changes and modifications may be made without departing from this invention and its broader aspects. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below.